1. Field of the Invention
The present invention generally relates to photosensitive polymers, and more particularly, the present invention relates to photosensitive polymers used in the manufacture of photoresist compositions, to photoresist compositions containing such photosensitive polymers, and to methods of forming photoresist patterns to be used in photolithography processes.
2. Description of the Related Art
As the integration density and complexity of semiconductor devices continue to increase, the ability to form ultra-fine patterns becomes more and more critical. For example, in 1-Gigabit or higher semiconductor devices, a pattern size having a design rule of 0.2 μm or less is needed. For this reason, in lithography processes, lower wavelength devices such as the ArF eximer laser (193 nm) have emerged as exposure light sources which are preferred over the more conventional and higher wavelength KrF eximer laser (248 nm).
However, compared with conventional (KrF) resist materials, resist materials which are suitable for use with the ArF eximer laser suffer from a variety drawbacks. The most serious problems relate transmittance and resistance to dry etching.
Almost all well-known ArF resist compositions contain (meth)acryl-based polymers. Among these polymers, a methacrylate copolymer having an alicyclic protecting group, which is expressed by the formula below, has been suggested (J. Photopolym. Sci. Technol., 9(3), pp. 509 (1996)): 
This polymer has an adamantyl group, which contributes to enhancing resistance to dry etching, and a lactone group, which improves adhesiveness, in its methacrylate backbone. As a result, the resolution of the resist and the depth of focus are improved. However, resistance to dry etching is still weak, and serious line edge roughness is observed after line patterns are formed from the resist layer.
Another drawback of the aforementioned polymer is that the raw material used to synthesize the polymer is expensive. In particular, the manufacturing cost of a polymer having a lactone group, which is introduced to improve adhesiveness, is so high that its practical use as a resist is difficult.
As another conventional resist composition, a cycloolefin-maleic anhydride (COMA) alternating polymer having the following formula has been suggested (J. Photopolym. Sci. Technol., Vol. 12(4), pp. 553 (1999), and U.S. Pat. No. 5,843,624): 
In the production of a copolymer, such as a COMA alternating polymer having the formula above, the production cost of raw material is relatively inexpensive, but the yield of the polymer sharply decreases. In addition, the transmittance of the polymer is very low at a short wavelength region, for example at 193 nm. The synthetic polymers have in their backbone the alicyclic group, which exhibits prominent hydrophobicity, and thus the adhesiveness to neighboring material layers is very poor.
The copolymer has a glass transition temperature of 200° C. or more due to the structural characteristic of the backbone. As a result, it is difficult to carry out an annealing process for eliminating free volume from the resist layer formed of the polymer, and accordingly the resist layer is influenced by ambient conditions which can cause, for example, a T-top profile of corresponding resist patterns. In addition, the resist layer itself becomes less resistant to ambient conditions during post-exposure delay, so that many problems can occur during subsequent processes with respect to the photoresist layer.
To improve the resolution of the resist layer, the polymer system may be charged with a polar group. In recent years, a technique of introducing a lactone group into a methacrylate monomer having an alicyclic protecting group, using the following alicyclic compounds with a lactone group, has been suggested so as to enhance the resistance to dry etching (J. Photopolym. Sci. Technol., Vol. 13(4), pp. 601 (2000), and Japanese Patent Laid-open No. hei 12-26446): 
Unfortunately, the yield of the monomer having the above formula is so low as to substantially increase manufacturing costs.
As described above, drawbacks of conventional ArF resist structures often relate to dry-etch resistance, LER and e-beam shrinkage. In particular, line-edge roughness (LER) and e-beam shrinkage are considered the most serious problems.
Environmentally stable chemically amplified photoresists require post bakes at temperatures close to the Tg of the bulk polymer in order to minimize the free volume (dynamic volume). As a result, Tg is an important property of polymer materials. At the vicinity of the Tg, the molecules of the photoresist materials become relatively stable from an initial disordered state, resulting in a reduction of free volume contained in the photoresist polymers. In the end, the photoresist film becomes hard and dense (annealing effect).
Film densification in this manner prevents contaminants from absorbing into the photoresist layer where they would otherwise interfere with a photosensitive acid generator and cause defective patterns, thus increasing image stability with respect to a delay between exposure and post-exposure bake (PEB). As a result, T-top profiles caused by contaminants or CD variation due to the diffusion of acid by post-exposure delay (PED) is reduced (ESCAP concept).
Annealing of the photoresist film advantageously reduces shrinkage from e-beam exposure because the photoresist film becomes relatively hard and dense. The robustness of annealed resists, which is presumably due to the reduced free volume, allows manipulation of the bake conditions for optimal performance.
Unfortunately, while annealing of most KrF resist materials is a relatively simple matter, annealing of known ArF resist materials is problematic in view of their substantially higher Tg. That is, the ArF photoresist can decompose at temperatures which are high enough to vaporize unwanted organic solvents and to harden the photoresist film. In other words, the temperatures needed to achieve the advantageous effects of annealing are too high to avoid decomposition of the ArF photoresist film.
Accordingly, there is a desire for ArF photoresist compositions which exhibit relatively low glass transition temperatures.